Add `compute_source_document_count` on `ReshardExecutor` — the reshard
backfill denominator (bf-2ynu5, AC #1). It delegates to the shared
`crate::index_stats::aggregate_index_stats`, reusing the executor's
existing `http_client` field as the reqwest transport, and returns
`total_documents` (reduced via `max` across source-node replicas).
Pure additive: no change to `start_backfill`, `advance_backfill`, or
persisted `ReshardOperation`. Wiring into `start_backfill` is a sibling
bead. `cargo build -p miroir-core` green.
Co-Authored-By: Claude <noreply@anthropic.com>
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